Brake systems for machines are generally known. Machines, such as wheel loaders, backhoe loaders, on-highway trucks, and off-highway trucks, have a number of wheels typically mounted in pairs on axles. Such machines also have a brake system which may include a service brake and a parking brake. The service brake, such as a drum brake, disc brake, or other known type, is associated with at least one wheel or axle to slow or stop a traveling machine. The parking brake, also of a known type, is associated with at least one wheel or axle to hold a stopped machine in position. Controls for the service and parking brakes are typically provided in a cab of the machine.
It is common for an operator to test the brake capacity of the service and/or parking brake prior to or during use of the machine. For example, the capacity of the service brake may be tested to ensure that the machine can safely stop when in motion in a variety of typical travel conditions and terrains. Similarly, the capacity of the parking brake may be tested to ensure that the machine can be safely held in place on almost any terrain.
In machines having a mechanical drivetrain, brake holding capacity can be tested by placing the drivetrain in a low torque mode. The mechanical drivetrain typically includes a transmission and torque converter so that higher gears may produce lower torque. Accordingly, when the transmission is placed in a high gear, typically third or fourth gear for many machines, it may produce a lower torque. To test brake capacity, the operator may engage either the service brake or the parking brake, place the transmission in a higher gear, and attempt to drive through the applied brake. If the brake holds the machine in place during this test, it may indicate that the brake capacity is sufficient for most operating conditions. With this method, however, it is difficult to accurately test the brake capacity as the higher gears may still produce a torque that exceeds test requirements.
A similar brake test is not possible for some machines having a hydrostatic drive. These machines have hydrostatic drives that produce full torque in all ranges, and therefore it is not possible to place them in a low torque condition by shifting into a higher gear. Additionally, while other machines may have hydrostatic drives capable of operating in a lower torque condition, brake capacity testing of those machines suffers from the same imprecision as the mechanical drivetrain test noted above.